Synchronization restoring system

ABSTRACT

Synchronization of the drive bars of a servo transfer system is safely restored, while avoiding mechanical damage as much as possible. 
     A synchronization position C for all the drive bars is determined from the respective present positions B of the drive bars and the respective deviations of the present positions B from the synchronization position C are determined. Then, servo motors for all the drive bars are so controlled that the drive bars move the distances corresponding to their respective deviations within the same period of time.

TECHNICAL FIELD

The present invention relates to a synchronization restoring systemwhich is suited for use in a servo transfer system in which amultiplicity of drive bars are driven by servo driving mechanisms tomove the feed bars in the direction of a lift axis, and which restoresthe drive bars to synchronization in the event that they losesynchronization.

BACKGROUND OF THE INVENTION

There are known transfer presses which include a transfer system forconveying workpieces to be pressed through a series of work stations intimed relation with performance of a series of pressing operations.Typically, the transfer system comprises a pair of feed bars juxtaposedso as to extend in a workpiece transferring direction and cross barseach spanned between these feed bars. The transfer system conveysworkpieces held by vacuum caps by vacuum adsorption, these vacuum capsbeing attached to the respective cross bars. Alternatively, the transfersystem conveys workpieces gripped at both sides by fingers attached tothe feed bars. In this case, the pair of feed bars performtwo-dimensional or three-dimensional movement to transfer an individualworkpiece from one station where a set of dies are disposed to the nextadjacent station where another set of dies are disposed.

The most typical method for driving the feed bars is a mechanicaldriving method in which the feed bars are driven, being linked to thepress system with a cam and linkage mechanism. This method howeverreveals the disadvantage that adjustment at the time of die replacementis extremely difficult and therefore the individual driving method (suchas disclosed in Japanese Patent Laid-Open Publication No. 6-218458(1994)) becomes prevailing recently according to which the feed bars aredriven with motors (servo motors) different from the motor for the presssystem. Transfer presses having a transfer system (servo transfersystem) driven by the individual driving method has the advantage thatthe transfer system can be changed arbitrarily in its movement and has asimple configuration.

If the above transfer system encounters abnormal conditions such as whenexcessive load is imposed on a servo motor for one of the drive barsduring operation, the drive bars would lose synchronization and bebrought to an emergency stop. If such a malfunction occurs, all thedrive bars need to be moved from their respective stop positions to aspecified position in order to restore the drive bars tosynchronization. In this case, if all the drive bars are moved to thespecified position X at the same speed, the arrival times of therespective drive bars vary according to the distances of their stoppositions from the specified position X, as understood from FIG. 6. Insome cases, the movement of the drive bars to the specified positioncauses the rigid, feed bars Y to be distorted with an excessive force.

The present invention is directed to overcoming the above problem andone of the objects of the invention is therefore to provide asynchronization restoring system which is capable of safely restoringthe drive bars of a servo transfer system to synchronization, whileavoiding mechanical damage as much as possible.

It should be noted that the definition of the feed bars disclosed hereininclude lift beams such as described in Japanese Patent Laid-OpenPublication No. 6-218458, the lift beams having cross bars that aremounted thereto so as to be freely movable in a workpiece transferringdirection and being designed to ascend and descend but not to move inthe transferring direction.

DISCLOSURE OF THE INVENTION

The above object can be achieved by a synchronization restoring systemfor restoring drive bar synchronization in a servo transfer system whichcarries out movement of feed bars in the direction of a lift axis bydriving a multiplicity of drive bars with servo driving mechanisms, therestoring system comprising:

(a) synchronization position detecting means for determining asynchronization position for all the drive bars from the respectivepresent positions of the drive bars;

(b) deviation detecting means for determining the deviation of thepresent position of each drive bar from the synchronization positiondetermined by the synchronization position detecting means; and

(c) controller means for controlling servo motors for the drive barssuch that the drive bars respectively move the distances correspondingto their respective deviations determined by the deviation detectingmeans within the same period of time.

According to the invention, in the event that the transfer system stopsdue to a loss of synchronization in the drive bars, restoration ofsynchronization is carried out in the following procedure: asynchronization position for all the drive bars is first determined fromthe respective present positions of the drive bars; the respectivedeviations of the present positions of the drive bars from thesynchronization position are determined; and then, the servo motors forthe drive bars are controlled to respectively move the associated drivebars the distances corresponding to their respective deviations withinthe same period of time. Thus, the drive bars start to move to thesynchronization position at the same time and arrive there at the sametime, so that the load to be imposed on the feed bars or on the motorsduring the restoration can be minimized. With this arrangement,restoration of the drive bars to synchronization can be performedwithout drawing the operator's attention, while avoiding mechanicaldamage as much as possible.

Preferably, the controller means of the invention controls theacceleration and deceleration of the servo motors according to cycloidcurves when the feed bars start and stop movement from thesynchronization position to a destination. If theacceleration/deceleration of the servo motors at the start and stop isperformed in accordance with straight lines, it causes abrupt changes inthe acceleration and deceleration speeds and as a result, the movementof the feed bars cannot be smoothly started and stopped unlessfollowability is sacrificed by reducing gain. In contrast with this, theacceleration/ deceleration in accordance with cycloid curves allows asmooth start and stop in the feed bar movement with less changes in theacceleration and deceleration speeds.

When the feed bars are in ascendant movement, the above synchronizationposition is determined to correspond to the highest position of thepresent positions of the drive bars, and when the feed bars are indescendent movement, the synchronization position is determined tocorrespond to the lowest position of the present positions of the drivebars. With this arrangement, the feed bars neither descend duringlifting operation nor ascend during lowering operation, which enables itto smoothly restore the drive bars to the synchronization positionwithout giving a feeling of disorder to the operator.

The synchronization position may be determined to correspond to theaverage of the levels of the present positions of the drive bars. Thisminimizes the moving distance of each drive bar at the time ofrestoration.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic general perspective view of a transfer pressconstructed according to one embodiment of the invention.

FIG. 2 diagrammatically depicts the system structure of the transferpress according to the embodiment.

FIG. 3 depicts an example of the motion pattern of a transfer system.

FIG. 4 is a flow chart of a program for restoring synchronization.

FIGS. 5(a) and 5(b) depict the manner in which synchronization isrestored according to the embodiment.

FIGS. 6(a) and 6(b) depict the manner in which synchronization isrestored according to prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, a synchronization restoring systemconstructed according to a preferred embodiment of the invention will behereinafter described.

FIG. 1 schematically depicts a general, perspective view of a transferpress constructed according to one embodiment of the invention, and FIG.2 diagrammatically depicts the system structure of this transfer press.As shown in FIG. 1, the transfer press according to this embodimentcomprises a press system 1 and a transfer system 2. The press system 1has a series of work stations which perform successive pressingoperations on each workpiece (not shown) whereas the transfer system 2is disposed within the press system 1, for transferring the workpiecesin a feeding direction A.

The press system 1 includes a press slide 3 which is reciprocablevertically by slide driving mechanisms spanned at the respective workstations. Upper dies are attached to the underside of the press slide 3while lower dies are so attached to moving bolsters 4 as to respectivelyface their corresponding upper dies so that pressing operations can beperformed on the respective workpieces placed between these dies. Eachof the slide driving mechanisms comprises a main motor 6 controlledaccording to signals from a press controller; a drive shaft 7 driven bythe main motor 6; a flywheel unit 8, clutches 9a, 9b and a brake (notshown) attached to the drive shaft 7.

The transfer system 2 has a pair of feed bars 11 juxtaposed so as toextend along the workpiece feeding direction A, being suspended fromabove by means of lift mechanisms attached to the press system 1. Eachlift mechanism includes a pinion 13 rotated by a servo motor 12 throughreduction gears 12A and a rack bar 14 which meshes with the pinion 13.The feed bars 11 are supported at the respective lower ends of the rackbars 14 and driven by the servo motors 12 so as to move vertically insynchronous relation with the movement of the press system 1. Balancecylinders 15 are disposed at positions adjacent the respective rack bars14, for maintaining a balance between the fluid pressure imposed on eachbalance cylinder 15 and the weight of the feed bars 11 etc. In thisembodiment, five servo motors 12 are aligned at equal intervals on bothsides so that the ten servo motors 12, in total, cooperate to lift orlower the pair of feed bars 11.

Supported on the undersides of the feed bars 11 are a plurality of crossbar carriers 16 which are spaced in the feeding direction A so as to bemovable both in the feeding direction A and in the direction opposite tothe feeding direction A. A cross bar 17 is spanned between each opposingpair of cross bar carriers 16 so as to extend in a directionperpendicular to the feeding direction A. The cross bars 17 respectivelyhave vacuum cups 18 attached thereto for adsorbing the workpieces.

The cross bar carriers 16 adjacent to each other in the feedingdirection A are coupled to each other by a coupling rod so that all thecross bar carriers 16 can simultaneously move both in the feedingdirection A and in the direction opposite thereto. The most upstreamcross bar carrier 16 attached to each feed bar 11 is connected to thedistal end of each cam lever 20 by means of a coupling rod 19. The baseend of each cam lever 20 adjoins to a feed cam 21 which is rotatable bypower delivered from the press system 1. The rotation of the feed cams21 brings the cam levers 20 in rocking movement thereby moving the crossbar carriers 16 both in the feeding direction A and in the directionopposite thereto.

The angle of rotation of each drive shaft 7 is detected by a press angledetector (cam angle detector) 22 and according to this detected pressangle, each servo motor 12 is controlled by a transfer system controller23 through its servo amplifier (servo driver) 24. With this arrangement,the cross bar carriers 16 of the transfer system 2 reciprocate in thefeeding direction A and in the direction opposite thereto synchronouslywith the movement of the press system 1 so that the vacuum cups 18attached to the cross bars 17 successively transfer the adsorbedworkpieces to the respective work stations.

The servo motors 12 are each equipped with a position detector (encoder)26 for detecting the present position of the servo motor 12 and eachposition detector 26 generates a positional signal to input to atransfer system controller 23. The transfer system controller 23calculates the difference between present position data input from eachposition detector 26 and press angle data input from a press angledetector 22 and generates a movement command to the associated servomotor 12 through its servo amplifier 24 such that the difference becomesequal to zero.

It should be noted that there is provided an independent motor 25 forindependently driving the transfer system 2 and each drive shaft 7 isalso driven by this independent motor 25 through a clutch 9c.

The transfer system 2 is driven according to a specified motion patternin order to avoid the interference between the dies and the workpiecesbeing transferred by the transfer system 2. FIG. 3 shows atwo-dimensional motion pattern as one example of this motion pattern.According to this example, for moving onto the lower die of the previouswork station, the transfer system 2 is first lifted at the stand-bypoint R, and then moved to and lowered at the adsorbing point P. At theadsorbing point P, the transfer system 2 adsorbs the workpiece to liftout of the lower die of the previous work station in the direction of Zaxis and then conveys it in the direction of X axis to the positionabove the lower die of the next work station. To place the workpiece inthis lower die, the transfer system 2 is lowered to release theworkpiece at the releasing point Q. After releasing, the transfer system2 is lifted and then moved back downwardly to the stand-by point Rthereby terminating one cycle.

If an error occurs in such a transfer system 2 during operation, thetransfer system 2 may be brought to an emergency stop with asynchronization failure in the drive bars of the servo motors 12, asshown in FIG. 5(a). In this embodiment, in the event that such asynchronization failure occurs, synchronization is restored according tothe flow chart shown in FIG. 4. The flow of this synchronizationrestoring process will be described with reference to the flow chart.

S1 to S2: The present value (present level) of each drive bar is readand a check is made to determine whether all the drive bars are in asynchronous relation, that is, whether there is a difference between thelevels of the drive bars.

S3 to S5: If the drive bars are not synchronous, the drive barpositioned at the highest level (the highest drive bar) and the drivebar positioned at the lowest level (the lowest drive bar) are detected.Then, speed V for each drive bar is calculated using the equationV=Δs/Δt and a command value to be released to the servo amplifier 24 foreach drive bar is obtained from the speed V. It should be noted that Δsis the distance from the present position of each drive bar to theposition of the highest drive bar when the feed bars 11 are lifted, andΔs is the distance from the present position of each drive bar to theposition of the lowest drive bar when the feed bars 11 are lowered. Δtis the time required for moving the drive bars from their presentpositions B to a synchronization position C (see FIG. 5), which is apredetermined value common to all the drive bars. Thus, the greater thevalue Δs, the greater the calculated speed V.

S6 to S8: If either an Up button or Down button (both buttons serve as apress button for independent manual operation generally called as "JOGoperation") is depressed, the calculated, command values arerespectively set in the corresponding servo amplifiers 24 and the servomotors 12 are actuated. If neither the Up button nor Down button isdepressed, the program returns to step S1. Thus, when the Up button isdepressed, each drive bar moves to the level of the highest drive bar asshown in FIG. 5 and when the Down button is depressed, each drive barmoves to the level of the lowest drive bar, so that all the drive barsbecome synchronous. Since the value of Δt is constant, all the drivebars start and finish their movement at the same time.

S9 to S10: When all the drive bars have reached the synchronizationposition with the above-described process, or if it is detected that allthe drive bars are in a synchronous condition, the program proceeds tothe normal JOG operation and a predetermined Up or Down end is set as adestination for each drive bar. Then, a movement table for movement fromthe present position of the drive bars to the Up or Down end isgenerated such that the acceleration at the time of starting and thedeceleration at the time of stopping are controlled according to camcurves such as cycloid curves.

S11 to S13: If either the Up button or Down button is depressed, a tablecommand value is set in the servo amplifiers 24 and the servo motors 12are actuated. If neither the Up button nor the Down button is depressed,the program returns to step S9. After the servo motors 12 have beenactuated, the program stands by until the Up button or Down button isnext depressed. Lifting or lowering of the drive bars is synchronouslyperformed in this way. Possible shock which may occur when the drivebars start and stop in the lifting/lowering movement can be reduced,since the acceleration and deceleration at the time of the start andstop are designed to follow cycloid curves or similar curves.

In the foregoing embodiment, the level of the highest drive bar is setas the synchronization position when the feed bars are in liftingoperation and the level of the lowest drive bar is set as thesynchronization position when the feed bars are in lowering operation.In an alternative embodiment, the synchronization position may be set tocorrespond to the average of the present levels of the drive bars, andthe drive bars at higher positions than the average level are loweredwhile the drive bars at lower positions than the average level beinglifted, thereby to restore synchronization.

I claim:
 1. A synchronization restoring system which restores drive barsynchronization in a servo transfer system which carries out movement offeed bars in the direction of a lift axis by driving a multiplicity ofdrive bars with servo driving mechanisms, the restoring systemcomprising:(a) synchronization position detecting means for determininga synchronization position for all the drive bars from the respectivepresent positions of the drive bars; (b) deviation detecting means fordetermining the deviation of the present position of each drive bar fromthe synchronization position determined by the synchronization positiondetecting means; and (c) controller means for controlling servo motorsfor the drive bars such that the drive bars respectively move thedistances corresponding to their respective deviations determined by thedeviation detecting means within the same period of time.
 2. Asynchronization restoring system according to claim 1, wherein thecontroller means controls the acceleration and deceleration of the servomotors according to cycloid curves, when the feed bars start and stopmovement from the synchronization position to a destination.
 3. Asynchronization restoring system according to claim 1 or 2, wherein whenthe feed bars are in ascendant movement, said synchronization positionis determined to correspond to the highest level of the presentpositions of the drive bars, and when the feed bars are in descendentmovement, said synchronization position is determined to correspond tothe lowest level of the present positions of the drive bars.
 4. Asynchronization restoring system according to claim 1 or 2, wherein saidsynchronization position is determined to correspond to the average ofthe levels of the present positions of the drive bars.